The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
An aircraft is moved by one or several turbojet engine(s) each housed in at least one nacelle. The nacelle presents generally a tubular structure comprising an air inlet section upstream of the turbojet engine, a mid-section intended to surround a fan of the turbojet engine, a downstream section accommodating the thrust reversal means. The downstream section of the nacelle surrounds the gas generator of the turbojet engine which is terminated by an ejection nozzle located downstream of the turbojet engine. The air inlet section of the nacelle includes in particular a generally annular-shaped front lip which intercepts the inlet air flow of the nacelle which is directed toward a fan. A nacelle generally presents a structure comprising an upstream air inlet section, upstream of the engine, a mid-section intended to surround a fan of the turbojet engine, and a downstream section intended to surround the combustion chamber of the turbojet engine and accommodating a thrust reverser device.
Lengthening the front lip of the nacelle is in particular desired for aerodynamic reasons, in order to extend downstream the laminar air flowing area. Nonetheless, its lengthening is not without repercussions on the design of the rest of the nacelle. Particularly, the nacelle should present good mechanical rigidity performances in order to reduce its deformations when subjected to stresses during operation.
In the state of the art, there is a model of nacelle, illustrated for example in U.S. Pat. No. 8,197,191 in which the front lip is constituted by a single annular-shaped part and which is directly fastened on support partition walls inside the nacelle. Note also the use of an upstream partition wall which forms an annular volume behind the <<D>>-shaped front lip. The upstream (or front) partition wall is generally secured to the internal envelope of the inlet section of the nacelle by its front edge.
In this document, the nacelle inlet section of an aircraft turbojet engine presents a turbojet engine envelope including a front flank. The nacelle inlet section includes:                an inner acoustic part, substantially cylindrical and presenting an upstream edge and a downstream edge;        an external envelope comprising a lip presenting an internal edge and a substantially cylindrical outer portion presenting a downstream portion;        a downstream mounting flange configured for mounting the inlet section to a front flange of an envelope of the turbojet engine; and        a rear partition wall presenting a downstream end for securing the outer portion of the external envelope to the downstream mounting flange.        
As a result, the part which forms the front lip which limits the fresh air inlet of the nacelle presents in cross section a complex and large sized shape. In U.S. Pat. No. 8,267,642, there is defined an outer envelope of a nacelle inlet section which includes a substantially cylindrical internal portion and a rear partition wall. The outer envelope includes:                a front lip with fasteners on the internal portion; and        an outer envelope extending downstream from the lip and intended to be attached to the rear partition wall.        
In the document, the front lip and the outer envelope form one single piece.
Such an arrangement described in the state of the art aims mainly at two objectives. Such a structure allows resisting an event such as blade breakage of the turbojet engine fan mounted inside the nacelle. In addition, it allows for savings in material mass and in complexity of parts in comparison with the older state of the art wherein the outer envelope and the front lip of the inlet section of the nacelle are constituted by separate parts.
However, in spite of their advantages, these arrangements require the adjunction of annular reinforcements, disposed inside the outer envelope. Furthermore, the thus formed single part has a large dimension and a significant volume. Its realization requires a complex and expensive tool.
Nonetheless, the solution of a lip that is integral with the outer envelope of the inlet section of the nacelle also encounters problems of adaptation with other requirements.
Among these, mention may be made in particular to the problem of maintenance. Indeed, in order to inspect the different systems such as the de-icing system or inspect the condition of the structure, we must to be able to access to the space comprised between the front and rear (or downstream) partition walls which is then inaccessible if the part that forms the lip and the outer envelope is in one single piece.
In order to provide access to the volume created below the external envelope of the air inlet section of the nacelle, a passage for an access hatch is provided, which considerably reduces the rigidity of the assembly and does not allow for one single piece combining the front lip and the outer envelope.
Finally, there is a particular structure of the internal envelope of the nacelle. This internal envelope includes an upstream portion (on the air inlet section side of the nacelle) constituted by an acoustic shroud and a downstream portion (on the thrust reverser side) comprising a fan casing. Both upstream and downstream portions are connected by an attachment flange. Nonetheless, a maintenance operation is made difficult through an advanced front lip integral with the downstream partition wall. Indeed, the skin constituted by this front lip integral with the downstream partition wall should be interrupted in order to arrange an access hatch, thereby weakening it, on the one hand, and breaking the aerodynamic continuity of the skin, on the other hand.